就reduce和concat而言，它可以在Java 8中表示为：

String fromFile = new BufferedReader(new   
InputStreamReader(inputStream)).lines().reduce(String::concat).get();

这是一个改编自org.apache.commons.io.IOUtils源代码的答案，适用于那些想要apache实现但不想要整个库的人。

private static final int BUFFER_SIZE = 4 * 1024;

public static String inputStreamToString(InputStream inputStream, String charsetName)
        throws IOException {
    StringBuilder builder = new StringBuilder();
    InputStreamReader reader = new InputStreamReader(inputStream, charsetName);
    char[] buffer = new char[BUFFER_SIZE];
    int length;
    while ((length = reader.read(buffer)) != -1) {
        builder.append(buffer, 0, length);
    }
    return builder.toString();
}

如果使用流读取器，请确保在结束时关闭流

private String readStream(InputStream iStream) throws IOException {
    //build a Stream Reader, it can read char by char
    InputStreamReader iStreamReader = new InputStreamReader(iStream);
    //build a buffered Reader, so that i can read whole line at once
    BufferedReader bReader = new BufferedReader(iStreamReader);
    String line = null;
    StringBuilder builder = new StringBuilder();
    while((line = bReader.readLine()) != null) {  //Read till end
        builder.append(line);
        builder.append("\n"); // append new line to preserve lines
    }
    bReader.close();         //close all opened stuff
    iStreamReader.close();
    //iStream.close(); //EDIT: Let the creator of the stream close it!
                       // some readers may auto close the inner stream
    return builder.toString();
}

编辑：在JDK7+上，可以使用trywithresources构造。

/**
 * Reads the stream into a string
 * @param iStream the input stream
 * @return the string read from the stream
 * @throws IOException when an IO error occurs
 */
private String readStream(InputStream iStream) throws IOException {

    //Buffered reader allows us to read line by line
    try (BufferedReader bReader =
                 new BufferedReader(new InputStreamReader(iStream))){
        StringBuilder builder = new StringBuilder();
        String line;
        while((line = bReader.readLine()) != null) {  //Read till end
            builder.append(line);
            builder.append("\n"); // append new line to preserve lines
        }
        return builder.toString();
    }
}

当输入流来自类路径资源（这似乎是一项流行任务）时，Guava提供了更短的高效自动关闭解决方案：

byte[] bytes = Resources.toByteArray(classLoader.getResource(path));

or

String text = Resources.toString(classLoader.getResource(path), StandardCharsets.UTF_8);

还有ByteSource和CharSource的一般概念，它们温和地负责打开和关闭流。

因此，例如，不要显式打开一个小文件来读取其内容：

String content = Files.asCharSource(new File("robots.txt"), StandardCharsets.UTF_8).read();
byte[] data = Files.asByteSource(new File("favicon.ico")).read();

或者只是

String content = Files.toString(new File("robots.txt"), StandardCharsets.UTF_8);
byte[] data = Files.toByteArray(new File("favicon.ico"));

使用java 9中支持的java.io.InputStream.transferTo（OutputStream）和ByteArrayOutputStream.toString（String），该字符串采用字符集名称：

public static String gobble(InputStream in, String charsetName) throws IOException {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    in.transferTo(bos);
    return bos.toString(charsetName);
}

以下是我经过一些实验后提出的最优雅、纯Java（无库）解决方案：

public static String fromStream(InputStream in) throws IOException
{
    BufferedReader reader = new BufferedReader(new InputStreamReader(in));
    StringBuilder out = new StringBuilder();
    String newLine = System.getProperty("line.separator");
    String line;
    while ((line = reader.readLine()) != null) {
        out.append(line);
        out.append(newLine);
    }
    return out.toString();
}